Now I should be able to pay for the product, but instead I'm redirected to a page that shows that my cart is empty - it looks like that: http://i.imgur.com/ZOXWKGq.png
And it is, indeed, empty - all I can do now is to add CMud to cart again and repeat all these steps I listed above, but it still ends in the same way.

I tried it using Firefox, Chrome and Internet Explorer - it looks like it is not a browser related problem but something on the backend.